The present invention relates to a semiconductor circuit of the type which may be used in equipment such as a power converting apparatus, a driving method of the semiconductor circuit and a semiconductor device.
A power converting apparatus comprises not only switching devices, such as MOSFETs and IGBTs (Insulated gate Bipolar Transistor), devices having a rectifying effect, such as diodes, and passive devices, such as capacitors, inductrances and resistors, but also wires which have a parasitic inductance. The power converting apparatus converts power by repeating the on and off states of the switching and rectifying devices, in which in on and off states, currents are flowing through and are cut off from the devices, respectively. As a result, a voltage rising abruptly to a value much higher than the power-supply voltage is applied to the switching and rectifying devices at the time the devices undergo transition from the on state to the off state due to the effect of parasitic inductance.
In order to prevent the devices from being damaged by an high voltage (a spike voltage) generated at the switching time, traditionally, devices having a breakdown voltage greater than a value obtained by estimating the magnitude of such a spike voltage are used, or, as an alternative, the spike voltage is eliminated by means of a snubber circuit. The use of devices having a high breakdown voltage entails not only a high cost but, also, an increased amount of incurred power loss, giving rise to an undesirable problem. On the other hand, the use of a snubber circuit for avoiding a spike voltage increases the number of components. As a result, the cost and the size of the power converting apparatus increase.
By the way, a spike voltage is generated at the switching time because a current that has been flowing through the switching device and the inductance abruptly decreases due to the switching operation of the switching device. Thus, if the abrupt decrease in current can be suppressed, the spike voltage can also be suppressed as well. In a typical method for suppressing an abrupt decrease in current, a Zener diode or an avalanche diode is employed in parallel to the switching device. When a voltage applied to an avalanche diode exceeds the breakdown voltage thereof, a current is allowed to flow through the avalanche diode, avoiding an abrupt decrease in current.
However, the use of such a diode gives rise to the following problem. When a current flows through an avalanche diode, the decrease in current flowing through a parasitic inductance disappears due to the fact that the resistance of the avalanche diode is all but zero once the avalanche diode has entered a breakdown state. The disappearance of the decrease in current, in turn, causes the voltage applied to the avalanche diode to decrease. As the voltage applied to the avalanche diode becomes lower than a voltage that causes the avalanche diode to enter an avalanche breakdown state, the avalanche diode enters an off state, trying to abruptly reduce a current flowing through the inductance. The attempt to abruptly reduce the current flowing through the inductance causes the avalanche diode to again enter an avalanche breakdown state in which a current can flow through it. That is to say, the voltage applied to the avalanche diode and the current flowing through it keep oscillating and the oscillation of the voltage and current is a cause of the generation of electromagnetic noise.
Another method besides the technique of using an avalanche diode is described, for example, in a document called EPE Journal, Vol. 4, No. 2, June (1994), pages 8 to 9. The method described therein is an example of techniques called dynamic clamping. According to the dynamic clamping technique, an avalanche diode is interposed between the collector and gate electrode of an IGBT, whereas a resistor is interposed between the gate and emitter electrode thereof. As the collector voltage exceeds the breakdown voltage of the avalanche diode, a current flows through the avalanche diode and the resistor, increasing the gate voltage. The increase in gate voltage causes a collector current to flow through the IGBT, preventing a large voltage from being applied to the device. In this case, none the less, a problem similar to that encountered in the technique of using an avalanche diode as described above also arises.
With the collector voltage exceeding the breakdown voltage of the avalanche diode, a voltage equal to the difference between the collector voltage and the avalanche-breakdown voltage is applied to the gate. That is to say, as the collector voltage exceeds the avalanche-breakdown voltage, the portion of the collector voltage above the avalanche-breakdown voltage is all applied to the gate.
In general, the IGBT has a collector current which is greatly changing due to a small variation in gate voltage so that, when the collector voltage exceeds the avalanche-breakdown voltage, the IGBT current increases abruptly. In the case of an IGBT having a breakdown voltage of several hundreds of volts and a rated current density of 200 A/cm2, for example, the saturated current density at a gate voltage of 15V reaches as much as several thousands of amperes per square cm. This implies that, when the collector voltage exceeds the breakdown voltage of an avalanche diode interposed between the collector and the gate by a potential of only 15V, the collector current can actually reach a value of several thousands of amperes. That is to say, an IGBT adopting the dynamic clamping technique exhibits a characteristic very similar to that of an avalanche diode wherein, at a certain voltage, the current abruptly increases. For this reason, this dynamic clamping technique also has the same problem as that encountered in the example of using an avalanche diode.
As described above, the techniques adopted in the conventional power converting apparatus for suppressing a spike voltage have problems of an increased amount of incurred power loss, a rising cost and generation of electromagnetic noise. It is thus an object of the present invention to provide a semiconductor circuit which is capable of solving these problems, a technique of driving the semiconductor circuit and a semiconductor device.
The semiconductor circuit provided by the present invention comprises a circuit incorporating at least a semiconductor device and an inductance connected to the circuit. The semiconductor circuit is used for controlling a current flowing through the circuit so that the current is turned on and off. A voltage is applied in a current blocking direction between the terminals of the circuit that includes the semiconductor device. When the blocking-direction voltage is greater than or equal to a first voltage value, but is smaller than or equal to a second voltage value, the current increases as the blocking-direction voltage increases. As the blocking-direction voltage further increases, exceeding the second voltage value, the current increases with an increase in blocking-direction voltage at a rate of increase higher than a rate of increase that prevails for values of the blocking-direction voltage greater than or equal to the first voltage value, but smaller than or equal to the second voltage value. For values of the blocking-direction voltage smaller than the first voltage value, only a leakage current flows, in substance, putting the circuit in a current cut-off state.
It should be noted that, in referring to a circuit including a semiconductor device, a wide range of circuits other than a circuit including semiconductor devices and other passive elements are implied. For example, a circuit including a semiconductor device can refer to a circuit comprising only semiconductor devices or a semiconductor module having semiconductor devices embedded in a case or having semiconductor devices embedded along with peripheral circuits in a case. In addition, in referring to a semiconductor device, a semiconductor switching device, the main control current of which can be controlled by a control signal, or a diode is meant. Furthermore, an inductance implies not only an inductance of a load, such as a motor, but also an inductance of a circuit wire. The meanings of the terms circuit, semiconductor device and inductance as explained above hold true for all configurations described below.
In addition, a circuit including a semiconductor device as described herein does not have to be a semiconductor circuit wherein an inductance is connected thereto and in which the flow of the main current can be controlled to on and off states. Instead, a circuit including a semiconductor device can be one of a variety of circuits, to which semiconductor devices are connected.
The present invention also provides a technique for controlling the semiconductor circuit whereby a control signal is supplied to a semiconductor switching device employed in the semiconductor circuit in accordance with a voltage applied to the main terminals of the semiconductor device in a current blocking direction so that, when the blocking-direction voltage is greater than or equal to a first voltage value, but is smaller than or equal to a second voltage value, a main current increases as the blocking-direction voltage rises, and, as the blocking-direction voltage further rises, exceeding the second voltage value, the main current increases with an increase in blocking-direction voltage at a rate of increase higher than a rate of increase that prevails for values of the blocking-direction voltage greater than or equal to the first voltage value, but smaller than or equal to the second voltage value. For values of the blocking-direction voltage smaller than the first voltage value, only a leakage current flows, in substance, putting the circuit in a current cut-off state.
In addition, the present invention also provides a semiconductor device wherein, when a voltage applied to main terminals of the semiconductor device in a current blocking direction is greater than or equal to a first voltage value, but is smaller than or equal to a second voltage value, a main current increases as the blocking-direction voltage rises, and, as the blocking-direction voltage further rises, exceeding the second voltage value, the main current increases with an increase in blocking-direction voltage at a rate of increase higher than a rate of increase that prevails for values of the blocking-direction voltage greater than or equal to the first voltage value, but smaller than or equal to the second voltage value. For values of the blocking-direction voltage smaller than the first voltage value, only a leakage current flows, in substance, putting the circuit in a current cut-off state. The actual structure of the semiconductor device having such a characteristic will become more apparent from a detailed description of preferred embodiments given later.
When the voltage applied to the circuit including the semiconductor device, the semiconductor circuit or the semiconductor device described above in a current blocking direction exceeds the first voltage value, a current flows in accordance with the magnitude of the blocking-direction voltage. This current prevents a current flowing through an inductance existing inside the circuit, or an inductance connected to the semiconductor device, from abruptly decreasing, gradually limiting the increase in voltage applied to the circuit or the semiconductor device. As the blocking-direction voltage is further increased, exceeding the second voltage value, an even larger current flows, abruptly limiting the increase in voltage applied to the circuit or the semiconductor device.
That is to say, a differential resistance for the range of the blocking-direction voltage between the first and second voltage values is greater than a differential resistance for the range of the blocking-direction voltage above the second voltage value. It should be noted that, by a differential resistance, a ratio of an infinitesimal change in voltage to an infinitesimal change in current is meant. For this reason, when a voltage is induced in the inductance in a switching operation of the semiconductor device or the like, the differential resistance for the range of the blocking-direction voltage between the first and second voltage values plays a role of absorbing energy accumulated in the inductance, and the differential resistance for the range of the blocking-direction voltage above the second voltage value plays a role of limiting a further increase in inductance voltage. As a result, electromagnetic noise caused by current and voltage oscillation and an excessive voltage leading to the destruction of the semiconductor device can be prevented from being generated.